This invention relates to curable compositions containing a compound having a thiazole functionality and a polymerizable functionality.
In the assembly of a semiconductor package to a printed wire boad, an integrated circuit chip is attached to a metal lead frame with adhesive and wire bonding, and the whole assembly then encapsulated in a molding resin. After encapsulation, the outer leads of the lead frame are attached to a printed circuit board. Any exposed copper surfaces on the lead frames or boards are subject to oxidation with exposure to air and routinely are coated with an antioxidant. Benzotriazole is an efficient corrosion inhibitor for copper and copper alloys in many environments. However, the presence of benzotriazole is suspected of interfering with the bonding process during the die attach, wire bonding, encapsulation, and final soldering operations in the manufacture of the semiconductor package and its attachment to a printed circuit board. Thus, there is a need for materials that will perform as a corrosion inhibitor and simultaneously promote adhesion.
This invention is a curable composition comprising a compound (hereinafter the thiazole compound) containing two chemistry segments: (1) a segment containing at least one thiazole functionality (including benzothiazole functionality), and (2) a segment containing at least one polymerizable functionality, such as, an electron donor functionality, an electron acceptor functionality, or an epoxy functionality.
The thiazole functionality on the thiazole compound will have the following structure: 
in which
R1 and R2 independently are H or a linear or branched alkyl or alkylene group, or a substituted or unsubstituted cyclic alkyl or alkylene group, or a substituted or unsubstituted aromatic group;
or R1 and R2 together form a substituted or unsubstituted cyclic alkyl or alkylene group;
or R1 and R2 together form a substituted or unsubstituted aromatic or heteroaromatic ring or fused ring having 3 to 10 carbon atoms within the ring structure, in which the heteroatoms may be N, O, or S;
in which the substituents on any ring are xe2x80x94OR3, xe2x80x94SR3, xe2x80x94N(R3)(R4), or an alkyl or alkylene group having 1 to 12 carbon atoms, in which R3 and R4 independently are an alkyl or alkylene group having 1 to 12 carbon atoms, preferably 1 to 4 carbon atoms;
and Z is any organic moiety that contains a polymerizable functionality.
Z can be polymeric, oligomeric, or monomeric, (for example, alkyl, cycloalkyl, aryl alkyl, alkenyl, cycloalkenyl, aryl alkenyl, or aromatic, and for example poly(butadiene), polyether, polyester, polyurethane, polyacrylic, polystyrene, polycarbonate, polysulfone).
The polymerizable functionality on Z will react with a complementary reactive functionality on any adhesive, coating, encapsulant, or other composition used in semiconductor manufacturing operations to immobilize the thiazole and prevent it from interfering with those manufacturing operations that are conducted proximate to metal surfaces.
Examples of polymerizable functionalities include electron donor groups, electron acceptor groups, and epoxy groups. Exemplary electron donor groups are vinyl ethers, vinyl silanes, compounds containing carbon to carbon double bonds attached to an aromatic ring and conjugated with the unsaturation in the aromatic ring, such as compounds derived from cinnamyl and styrenic starting compounds. Exemplary electron acceptor groups are acrylates, fumarates, maleates, and maleimides.
The thiazole compounds may be used as the main component of curable compositions, which will further comprise a curing agent and a filler.
Alternatively, the thiazole compounds of this invention may be added to adhesive, coating, encapsulant, or other curable compositions that come into contact with or that are required to bond to metal surfaces. As additives to retard oxidation and promote adhesion in curable compositions, they will be used in an effective amount to promote adhesion. In general, an effective amount will range from 0.005 to 20.0 percent by weight of the adhesive, coating, or encapsulant formulation.
In addition, such formulations will contain a polymerizable resin, optionally a curing initiator, and optionally a conductive or nonconductive filler.
Suitable polymerizable resins that may be used in the adhesive, coating, encapsulant or sealant formulations are known to practitioners in those arts. Examples of such resins include epoxies, electron donor resins, for example, vinyl ethers, vinyl silanes, and resins that contain carbon to carbon double bonds attached to an aromatic ring and conjugated with the unsaturation in the aromatic ring, such as compounds derived from cinnamyl and styrenic starting compounds; and electron acceptor resins, for example, fumarates, maleates, acrylates, maleimides, and thiol-enes (a compound resulting from the reaction of a thiol and a compound containing a carbon to carbon double bond).
Suitable curing agents are thermal initiators and photoinitiators present in an effective amount to cure the adhesive, coating, encapsulant or sealant formulation. In general, those amounts will range from 0.5% to 30%, preferably 1% to 20%, by weight of the total organic material (that is, excluding any inorganic fillers) in the formulation.
Preferred thermal initiators include peroxides, such as butyl peroctoates and dicumyl peroxide, and azo compounds, such as 2,2xe2x80x2-azobis (2-methyl-propanenitrile) and 2,2xe2x80x2-azobis(2-methyl-butanenitrile). A preferred series of photoinitiators is one sold under the trademark Irgacure by Ciba Specialty Chemicals. In some formulations, both thermal initiation and photoinitiation may be desirable: the curing process can be started either by irradiation, followed by heat, or can be started by heat, followed by irradiation.
In general, the formulations will cure within a temperature range of 70xc2x0 C. to 250xc2x0 C., and curing will be effected within a range of ten seconds to three hours. The actual cure profile will vary with the components and can be determined without undue experimentation by the practitioner.
The formulations may also comprise electrically or thermally conductive fillers or nonconductive fillers. Suitable conductive fillers are carbon black, graphite, gold, silver, copper, platinum, palladium, nickel, aluminum, silicon carbide, boron nitride, diamond, and alumina. Suitable nonconductive fillers are particles of vermiculite, mica, wollastonite, calcium carbonate, titania, sand, glass, fused silica, fumed silica, barium sulfate, and halogenated ethylene polymers, such as tetrafluoroethylene, trifluoroethylene, vinylidene fluoride, vinyl fluoride, vinylidene chloride, and vinyl chloride. If present, fillers generally will be in amounts of 20% to 90% by weight of the formulation.
In another embodiment, the thiazole compounds may be used to coat on exposed metal surfaces, such as the copper surfaces of a semiconductor device or printed circuit board. The metal surface may first be degreased, cleaned, polished or buffed. In this embodiment, the thiazole adduct typically is used at a concentration of 0.5% to 20% in any suitable solvent. Representative suitable solvents are water, ketones (such as, methyl ethyl ketone, methyl isobutyl ketone, acetone), alcohols, glycol ethers, esters, and toluene.
The metal substrate is immersed in the solution for a period of time sufficient to deposit an effective coating. Immersion times typically will range from one second to one hour, more typically one minute to 15 minutes, although shorter or longer times may be effective depending on the particular thiazole compound, solution strength, and solution temperature. In general, the solution bath will be at a temperature within the range of 15xc2x0 C. to 100xc2x0 C.
Alternatively, the thiazole compound in solution can be sprayed or painted onto the metal surface to be coated. The solution is typically air-dried from the surface, and then cured at an elevated temperature suitable for removing any remaining solvent and for effecting curing.